Method, apparatus and system for providing transmit diversity feedback during soft handoff

ABSTRACT

Embodiments of the present invention describe methods for increasing the amount of information available to a mobile transmit diversity transmitter during soft handoff. According to embodiments of the invention, a transmit diversity transmitter may determine substantially when it is within the range of at least two receivers. In such situations, the transmitter may use the power control signals from more than one receiver, for example, from the two most dominant receivers in order to focus the beamforming accordingly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/043,256, filed on Mar. 6, 2008, which claimsbenefit of U.S. Provisional Patent Application Ser. No. 60/906,829,entitled “Method, Apparatus and System for Providing Transmit DiversityFeedback During Soft Handoff” and filed Mar. 14, 2007, the entirety ofwhich are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to the field of wireless communicationsand more specifically to controlling transmit diversity parameters in amobile communication device in a vicinity of more than one base station.

BACKGROUND OF THE INVENTION

A mobile or modifying communication device may have multiple antennaelements that transmit signals to communicate information. A basestation or feedback communication device may extract information fromthe transmitted signals. Multiple antenna elements may enhance spatialor spectral efficiency, allowing for more users to be simultaneouslyserved over a given frequency band. The transmitted signals, however,propagate along different paths and may reach the receivingcommunication device with different phases that destructively interfere.It is generally desirable to reduce interference of transmitted signals.

US Patent Publication No. 2003/0002594, assigned to the assignee of thepresent application, the contents of which are hereby incorporatedherein by reference, describes using a power control signal, forexample, as provided by the power control bit of the CDMA protocol, as aquality indication signal.

SUMMARY OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention may improve performance of a mobiletransmit diversity communication device during soft handoff situations.For example, imperfections may occur when the transmitter attempts toform or focus a single beam toward two differently located receivers,thus achieving suboptimal solution for both locations. In someembodiments of the invention, the method, system and apparatus describedin US Patent Publication No. 2003/0002594 may be improved, supplementedor replaced by those describe herein. Embodiments of the presentinvention may operate to focus beam-forming at two or more base stationreceivers based on feedback received from such two or more feedbacksources.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a block diagram illustrating one embodiment of a communicationsystem in accordance with the present invention;

FIG. 2 is a schematic diagram of a device according to embodiments ofthe present invention, in which the transmitter may have at least twotransmit paths each controlled by respective independent diversityparameters; and

FIG. 3 is a schematic flow diagram of a method according to anembodiment of the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 is a block diagram illustrating one embodiment of a communicationsystem 100 that includes a mobile transmitter 110, also referred to as amodifying communication device, that that adjusts a nominal value of atransmit diversity parameter, for example, a phase difference and/or apower ratio between a signal transmitted on a first antenna 111 and asecond antenna 112. Although the embodiments described in the presentapplication are described as using two antennas, it will be recognizedthat the present invention is equally applicable to transmit diversitysystems and devices having more than two antennas.

According to the embodiment, transmitter may, for example, perturb asignal at a perturbation rate and transmit the signal to receivingcommunication device 120, also referred to as a feedback communicationdevice. Feedback communication device 120 may receive the transmitdiversity signal at antenna 121 and transmit/receive module 122, processthe received signal using processor 123, and transmit feedbackinformation that describes the signal quality as received by feedbackcommunication device 120. It will be recognized that there may be avariety of ways for the feedback device to provide this signal qualityindication. In one embodiment described herein, the power control bit(PCB) may be used as a signal quality indicator. In other embodimentsone or more feedback parameters, including antenna selection and/ordedicated transmit diversity feedback parameters, may be usedadditionally or alternatively. Modifying communication device 110 mayadjust a nominal value of at least one transmit diversity parameter at anominal value adjustment rate based on the feedback information.

According to the illustrated embodiment, network 100 operates to provideservices such as communication sessions. A communication session mayrefer to an active communication between endpoints, measured fromendpoint to endpoint.

Information is communicated during a communication session. Informationmay refer to voice, data, text, audio, video, multimedia, control,signaling, other information, or any combination of the preceding.

The information may be communicated in packets. A packet may comprise abundle of data organized in a specific way for transmission, and a framemay comprise the payload of one or more packets organized in a specificway for transmission. A packet-based communication protocol such asInternet Protocol (IP) may be used to communicate the packets.

Network 100 may utilize communication protocols and technologies toprovide the communication sessions. Examples of communication protocolsand technologies include those set by the Institute of Electrical andElectronics Engineers, Inc. (IEEE) 802.xx standards, InternationalTelecommunications Union (ITU-T) standards, European TelecommunicationsStandards Institute (ETSI) standards, Internet Engineering Task Force(IETF) standards, or other standards.

Devices of network 100 may use any suitable multiple access technology,for example, a code division multiple access (CDMA) technology.According to one embodiment, network 10 may operate according to a CDMA2000 telecommunications technology that uses a single CDMA channel. Asan example, a CDMA 2000 high rate data packet technology, such as theEvolution Data Only (EvDO) technology may be used.

Network 100 may comprise any suitable communication network. Acommunication network may comprise all or a portion of a public switchedtelephone network (PSTN), a public or private data network, a local areanetwork (LAN), a metropolitan area network (MAN), a wide area network(WAN), a global computer network such as the Internet, a wirelessnetwork, a local, regional, or global communication network, anenterprise intranet, other suitable communication link, or anycombination of the preceding.

A component of network 100 may include logic, an interface, memory,other component, or any suitable combination of the preceding. “Logic”may refer to hardware, software, other logic, or any suitablecombination of the preceding. Certain logic may manage the operation ofa device, and may comprise, for example, a processor. “Interface” mayrefer to logic of a device operable to receive input for the device,send output from the device, perform suitable processing of the input oroutput or both, or any combination of the preceding, and may compriseone or more ports, conversion software, or both. “Memory” may refer tologic operable to store and facilitate retrieval of information, and maycomprise a Random Access Memory (RAM), a Read Only Memory (ROM), amagnetic drive, a disk drive, a Compact Disk (CD) drive, a Digital VideoDisk (DVD) drive, a removable media storage, any other suitable datastorage medium, or a combination of any of the preceding.

Communication network 100 may include one or more modifyingcommunication devices 110 and one or more feedback communication devices120 that communicate via a wireless link 130. Either or both ofcommunication devices 110 and 120 may be any device operable tocommunicate information via signals with one or more other communicationdevices. For example, either of communication devices 110 or 120 maycomprise a mobile subscriber unit or a base station. A subscriber unitmay comprise any device operable to communicate with a base station, forexample, a personal digital assistant, a cellular telephone, a mobilehandset, a computer, or any other device suitable for communicatingsignals to and from a base station. A subscriber unit may support, forexample, Session Initiation Protocol (SIP), Internet Protocol (IP), orany other suitable communication protocol.

A base station may provide a subscriber unit access to a communicationnetwork that allows the subscriber unit to communicate with othernetworks or devices. A base station typically includes a basetransceiver station and a base station controller. The base transceiverstation communicates signals to and from one or more subscriber units.The base station controller manages the operation of the basetransceiver station.

In some embodiments of the invention, the feedback communication device120 may be a base station, and the modifying communication device 110may be a subscriber unit.

Either or both of communication devices 110 or 120 may include one ormore antenna elements, where each antenna element is operable toreceive, transmit, or both receive and transmit a signal. Multipleantenna elements may provide for a separation process known as spatialfiltering, which may enhance spectral efficiency, allowing for moreusers to be served simultaneously over a given frequency band.

A communication link between communication devices 110 and 120 such aswireless link 130 may be a radio frequency link that is cellular innetwork organization. Wireless link 130 may be used to communicate asignal between communication devices 120 and 110.

As described more fully below, according to embodiments of the presentinvention, modifying communication device 110 may include a processor114 and a transmit/receive module 113 that calculate and produce one ormore signals for transmission over at least first and second antennas111 and 112.

Feedback communication device 120 may include a processor 123 andtransmit/receive module 122 that generate and transmit a feedback signalthat indicates the quality of the modified signal as received at thefeedback communication device 120. Modifying communication device 110may then modify the transmit signal in accordance with feedbackinformation corresponding to the feedback signal.

According to one embodiment, modifying a signal may refer to modifying asignal feature. A transmission signal feature, or in some embodiments ofthe invention, a transmit diversity parameter, may refer withoutlimitation to any feature of the transmission, for example, relativephase, relative amplitude, relative power, absolute power, frequency,timing, other suitable signal feature that may be modulated, or anycombination of the preceding. Relative phase may refer to the phasedifference between the phase of a first signal of a first transmitantenna element and the phase of a second signal of a second transmitantenna element. Relative power may refer to the ratio between the powerof a first signal of a first transmit antenna element and the power of asecond signal of a second transmit antenna element, which ratio may bedefined on a linear or logarithmic scale. Relative amplitude may referto the ratio between the amplitude of a first signal of a first transmitantenna element and the amplitude of a second signal of a secondtransmit antenna element. Absolute power may refer to the total powertransmitted by all antennas of modifying communication device 110.According to one embodiment, modifying a signal may be described asadjusting a nominal value of a transmit diversity parameter. Asdescribed more fully herein, according to an embodiment of theinvention, modulation of a transmit diversity parameter during aperturbation cycle may comprise transmitting using a transmit diversityparameter deviating from the nominal value in a first direction during afirst portion of the perturbation cycle and then transmitting using atransmit diversity parameter deviating from the nominal value in asecond direction during a second portion of the perturbation cycle.

According to one embodiment of operation of the invention, modifyingcommunication device 110 may modify a signal by perturbing the signal.Perturbing a signal may refer to modulating a signal feature of thesignal in relation to a nominal value of the signal, for example,modifying the signal feature in a first direction for a first feedbackinterval, and in a second direction for another feedback interval. Aperturbation cycle may refer to a first modulation in a first directionand a second modulation in a second direction. In some embodiments ofthe invention, a perturbation cycle may comprise a different, e.g.,longer or more complex, sequence of modulations. As an example withrespect to phase, a perturbation may include modulating the phasedifference in a first direction, and modulating the phase difference ina second direction. If the feedback information provided by the feedbackcommunication device 120 indicates an improvement in the signal receivedusing one perturbation modulation direction compared to the signalreceived using the other perturbation modulation direction, the nextnominal value adjustment may be made in the improved direction in anamount less than or equal to the modulation.

According to embodiments of the invention, the nominal value of atransmit diversity parameter may be perturbed at a first rate,designated the perturbation rate, and the nominal value of the transmitdiversity parameter may be adjusted at a second rate, designated thenominal value adjustment rate. The perturbation rate and the nominalvalue adjustment rates may be the substantially the same or they may bedifferent, and each one may be substantially the same or different thanthe feedback rate.

In an embodiment of the present invention, feedback communication device120 may transmit to modifying communication device 110 a power controlsignal, for example, one or more power control bits, or any type orgroup of power control signals may be used. A power control signal mayindicate to the modifying communication device 110 that it should raiseor lower its power. For example, based on the parameter being modulated,a “quality up” outcome value may instruct modifying communication device110 to increase the total power of its transmitted signal, and a“quality down” outcome value may instruct modifying communication device110 to decrease the total power. An outcome value may comprise, forexample, a power control bit of a CDMA power control signal, for which“0” represents a command to increase power and a “1” represents acommand to decrease power. Thus, for example, a bit value of “1”requesting a decrease in power may indicate “quality up,” and a bitvalue of “0” requesting an increase in power may indicate “qualitydown.” Some CDMA or W-CDMA protocols may allow for additional outcomes,for example, no change in power.

A transmitter may simultaneously receive power control signals frommultiple receivers, for example, when the transmitter is a mobile devicewithin range of a plurality of base stations. This is typically referredto as a soft handoff situation. These power control signals may providethe transmitter with conflicting instructions, e.g., one base stationmay indicate “quality up” and another may indicate “quality down.”However, the transmitter may only respond to one of the conflictinginstructions. The transmitter decision is therefore based on theaggregate power control signal, which may indicate “quality up” if atleast one received power control signal is a “quality up” and otherwise“quality down”. For example, the aggregate power control signal may beimplemented as the logical OR value of all received power controlsignals. That is, if all received power control signals indicate “0”,then all have received a degraded signal and are requesting increase inpower, and accordingly, the aggregate power control signal will be “0”and the transmitter will increase power. In another case, if at leastone power control signal indicates “1”, then at least one base stationhas received an improved signal and is requesting decrease in power, andaccordingly, the aggregate power control signal will be “1” and thetransmitter will decrease increase power.

According to embodiments of the present invention, transmit diversitytransmitters may obtain feedback information from multiple power controlsignals, whether alone or in addition to other feedback signals andparameters.

In one embodiment of the invention, the transmitter may first ascertainthat it is substantially within the range of more than one receiverproviding a meaningful power control signal. In some embodiments of theinvention, the determination that the transmitter is in soft handoff maybe made statistically over a number of power control signals receivedover a test period, for example, 100 ms. For example, the transmittermay enter soft handoff mode when at least two power control signalstreams during the test period each include a number of “quality down”signals greater than a threshold value. The threshold value may bedefined in relative or absolute terms. For example, the thresholdcondition for soft handoff mode may require that more than 25% of powercontrol signals received from each of at least two feedback sources be“quality down”. In another embodiment, the threshold may be at least30%, 40% or 50%. In an embodiment of the invention, the threshold may bedefined in comparative terms, for example, when the number of “qualitydown” signals from two feedback sources are approximately equal, forexample, when their number of “quality down” signals for a test periodare within no more than approximately 10%, 20%, or 30% of each other. Itwill be recognized that other determinations may be made forestablishing a soft handoff situation, depending, for example, onwhether the beamforming is to be focused more strongly at the weakerbase station or at the more dominant base station.

It will be understood in the context of the present application, thatthe determination made by the mobile transmitter that it is in a softhandoff situation is merely shorthand for a situation in which thedevice is within range of receiving meaningful feedback from more thanone base station. It is possible within the scope of the presentinvention that a transmitter may determine that it is in a soft handoffsituation and proceed accordingly even when service is not actuallybeing handed off from one base station to another, for example, when themobile transmitter is stationary and equidistant (power-wise) from twobase stations, in which case service may not be handed off, but thedevice may operate in soft handoff mode. For example, a soft handoffalgorithm may include a hysteresis component in order to avoid excessivehandoffs. In such a system, even though the soft handoff is not actuallyperformed, the presence of at least two meaningful base station signalsmay be used in connection with the present invention.

When a determination is made that the transmitter is in soft handoff, itmay regard each of the power control signals as an individual feedbackstream, and direct its beamforming to both receivers.

FIG. 2 depicts a device 200 according to embodiments of the invention,in which the transmitter may have at least two transmit paths eachcontrolled by respective independent diversity parameters. A signal fortransmission may be received at input 201. The power of the signal maybe divided at power divider 210. In one embodiment of the invention, thepower divider may optionally be a variable power divider, wherein theproportions of the power dividing may be controlled by input signal 211,which may provide a relative strength of a signal to be transmitted overone set of antennas or the other. For example, for an input signal 211of 50%, half of the signal power will be diverted each of the outputs ofpower splitter 210. The determination of how much power should go toeach of the outputs may be based on a statistical analysis of thereceived feedback signals, e.g., power control signals. As describedabove, in one embodiment of the invention, the transmitter may ascertainthat it is in soft handoff based on the number of “quality down” signalsin a test period. The input to power splitter 210 may be proportional tothe results of the results of the test period analysis. For example, ifin the soft handoff determination, it is ascertained that one feedbacksource, e.g., a base station, produces 30% “quality down” power controlsignals, and another feedback source produces 60% “quality down” powercontrol signals, then the input to power divider 210 may be ⅓, i.e.,30/(30+60) to one output and ⅔, i.e., 60/(30+60) to the other. It willbe recognized that in implementations of the invention, for example,where input signal 211 is a digital signal, there may be roundingeffects due to quantization. Algorithms for producing input signal 211based on the feedback signal are not described in detail herein,however, any number of mobile transmit diversity algorithms may be used.Similarly, the sub-system for producing the input signal 211 is notdescribed, however, it may be a processor, a memory, or any suitablesub-system.

Each of the outputs of power divider 210 may then be treated with adifferent diversity control parameter based on a respective powercontrol signal. Thus, in the example above, where the first and secondpower control signals have 30% and 60% “quality down” values,respectively, the output branch receiving ⅓ of the power may becontrolled based on the signal providing the 30% “quality down” signals,and the output branch receiving ⅔ of the power may be controlled basedon the signal providing the 60% “quality down” signals.

It will be understood that in order to simplify decorrelating thesignals at the receiver, one of modules 225 and 226 may have a delayintroduced thereto, for example, by delay element 220. In an embodimentof the invention, the delay introduced may be at least one chip. Otherembodiments may employ other methods for decorrelating the signals, forexample, by transmitting using three antennas, for example, where oneprimary antenna may be driven with a half-power amplifier, and thesecond and third antennas may be driven using quarter-power amplifiers.Other fractional power amplifiers may be used, for example, each antennamay be driven using a one-third-power amplifier.

The signal carried on each output branch of power divider 210 may betreated separately. For example, each power-divided signal may beprovided to modules 225 and 226, respectively for processing. Module 225may include, for example, power splitter 230, and phase shifter 240.Power splitter 230 may split the signal into first and second transmitbranches. Phase shifter 240 may produce a phase shift on one of thebranches controlled by input 240A. Similarly, module 226 may include,for example, power splitter 231, and phase shifter 241. Power splitter231 may split the signal into first and second transmit branches. Phaseshifter 241 may produce a phase shift on one of the branches controlledby input 241A. It will be recognized that during soft handoff, phaseshifter control inputs 240A and 241A may be derived separately from eachof the two power control feedback signals, respectively. It will beunderstood that there are a variety of methods and algorithms forcontrolling a diversity parameter based on a power control signal, asdescribed by other patent applications of the present assignee.

After producing the two diversity signals by each of modules 225 and226, the signals may be combined by adders 250 and 251, amplified bypower amplifiers 260 and 261, and transmitted on antenna elements 270and 271, respectively. It will be understood that in some embodiments ofthe invention, it may be advantageous to place power amplifiersadditionally or alternatively before the adding elements.

In some embodiments of the invention, when not in soft handoff mode, atransmitter may operate based on the aggregate power control signal, oraccording to a single most dominant power control signal. It will beevident that this aspect of the invention may be implemented in anynumber of ways. In an embodiment of the invention, for example, wherethe power divider 210 is a variable power divider, when not in handoffmode, the input to the variable power divider may be set to provide 100%of power to a single operating branch of the circuit of FIG. 2, forexample, the branch lacking the delay element. In another embodiment,when not in soft handoff mode, the delay element 220 may be bypassed byswitch 221, and the phase shifting elements 240 and 241 may be set toreceive the same input values, based on the aggregate power controlsignal, or according to a single most dominant power control signal.

Reference is made to FIG. 3, which is a schematic flow diagram of amethod 300 according to an embodiment of the present invention. Adetermination may be made whether the mobile device is in a soft-handoffsituation, e.g., whether it is in the vicinity of two usable feedbacksignals (310). This determination may be made according to any of thecalculations discussed above, or any suitable technique for determiningwhether feedback signals from two feedback communication devices may beused. If the device is in a soft-handoff situation, then the device mayreceive signals from first and second feedback devices (320). A datasignal is split into first and second transmit paths (330). A value of atransmit diversity parameter may be controlled for each transmit pathbased on a feedback signal from a respective feedback device (340). Thefirst and second transmit path signals may be transmitted (350). It willbe recognized that additional steps may be added, for example, adding atransmit delay, or, for example, recombining transmit paths as describedabove.

Certain embodiments of the invention may include none, some, or all ofthe above technical advantages. One or more other technical advantagesmay be readily apparent to one skilled in the art from the figures,descriptions, and claims included herein.

The previous description of the embodiments is provided to enable anyperson skilled in the art to make or use the invention. While theinvention has been particularly shown and described with reference toembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention. For example, anymobile communication protocol may be used, for example, CDMA or othertypes of protocols. For example, the communication devices similar tothose described above can be used with time-division multiple access(TDMA) or frequency-division multiple access (TDMA) protocols. Such aTDMA protocol can include, for example, the Global Systems for MobileCommunications (GSM) protocol.

Note that although the tuning of a communication device is describedthrough the use complex weighting, in other embodiments other types ofcontrol signals can tune the communication device. In other words, thetuning of a communication device through the use such control signalsneed not be limited to information about varying the magnitude and phaseof the signal. For example, the control signals can carry information tovary the magnitude, phase, frequency and/or timing of the signalassociated with each antenna element.

Embodiments of this invention may apply to any transmit diversitycontrol method. It will be understood that the methods discussed hereinmay be integrated with any transmit diversity control algorithm. It willfurther be understood that the present invention may be implemented as astand-alone processing module, or may be integrated into a transmitdiversity control processor, algorithm, or signal path circuitry.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. A method of controlling a mobile communication device comprising:receiving at the mobile communication device first and second usablefeedback signals transmitted by respective first and second basestations in a vicinity of the mobile communication device; determining afirst transmit diversity parameter based on the first feedback signal;determining a second transmit diversity parameter based on the secondfeedback signal; transmitting a modified transmit diversity signal onfirst and second antennas of the mobile communication device, saidmodified transmit diversity signal having a modified transmit diversityparameter based on said first and second diversity parameters.
 2. Themethod of claim 1, wherein the modified transmit diversity parameter isbased on a weighted combination of the first and second transmitdiversity parameters.
 3. The method of claim 2, wherein the weights ofthe first and second transmit diversity parameters in the weightedcombination are based on a comparison of the first and second usablefeedback signals.
 4. The method of claim 3, wherein the comparison ofthe first and second usable feedback signals is a comparison of thenumber of signals received from the first base station and the secondbase station requesting that the mobile communication device reducetransmission power.
 5. The method of claim 1, wherein the transmitdiversity parameter is a phase difference.
 6. A mobile communicationdevice comprising: a receiver to receive first and second usablefeedback signals transmitted by respective first and second basestations in a vicinity of the mobile communication device; a processorto determining a first transmit diversity parameter based on the firstfeedback signal and a second transmit diversity parameter based on thesecond feedback signal; a transmitter to transmit a modified transmitdiversity signal on first and second antennas of the mobilecommunication device, said modified transmit diversity signal having amodified transmit diversity parameter based on said first and seconddiversity parameters.
 7. The mobile communication device of claim 6,wherein the modified transmit diversity parameter is based on a weightedcombination of the first and second transmit diversity parameters. 8.The mobile communication device of claim 7, wherein the weights of thefirst and second transmit diversity parameters in the weightedcombination are based on a comparison of the first and second usablefeedback signals.
 9. The mobile communication device of claim 8, whereinthe comparison of the first and second usable feedback signals is acomparison of the number of signals received from the first base stationand the second base station requesting that the mobile communicationdevice reduce transmission power.
 10. The mobile communication device ofclaim 6, wherein the transmit diversity parameter is a phase difference.